The small computer system interface (SCSI) is a specification (ANSI standard X3.131-1986) for a peripheral bus and command set that defines a high performance peripheral interface that distributes data between a host or controller and a number of peripherals, independently of the host or controller. The SCSI interface works by masking the internal structure of the peripherals from the host computer and uses an eight-port bus that can accommodate either single or multiple-host systems.
The SCSI bus typically has a total of eighteen (18) defined signal lines. Nine (9) are used for control and nine (9) are used for data (eight (8) data lines and one (1 parity line). The data lines are bi-directional and transfer data, commands, status and message information. The control signals and the bus phases determine when and in what direction data is transferred.
The SCSI bus supports two electrical specifications, so-called single-ended and differential. The single-ended version uses TTL logic levels (0 to 5 volts), while the differential uses EIA RS-485 signals to allow longer cables. The present invention is particularly well suited for the single-ended electrical specification.
The SCSI bus is defined as including fifty (50) lines or signals, of which, as mentioned, nine (9) are data signals (including parity) and nine (9) are control signals. The following table contains the pin assignments for the single-ended signals. For the single-ended signals, all odd numbered lines plus the lines numbered twenty (20) twenty-two (22), twenty-four (24), twenty-eight (28), thirty (30), and thirty-four (34) are connected to ground on the controller and pin twenty-five (25) is left open.
TABLE I ______________________________________ Pin Mnemonic Signal Driven By ______________________________________ DBO Data Bus Line 0 Initiator/Target 4 DBl Data Bus Line 1 Initiator/Target 6 DB2 Data Bus Line 2 Initiator/Target 8 DB3 Data Bus Line 3 Initiator/Target 10 DB4 Data Bus Line 4 Initiator/Target 12 DB5 Data Bus Line 5 Initiator/Target 14 DB6 Data Bus Line 6 Initiator/Target 16 DB7 Data Bus Line 7 Initiator/Target 18 DBP Data Bus Parity Initiator/Target 26 TERMPWR Terminator Power Any device 32 ATN Attention Initiator 36 BSY Busy Initiator/Target 38 ACK Acknowledge Initiator 40 RST Reset Any device 42 MSG Message Target 44 SEL Select Initiator/Target 46 C/D Control/Data Target 48 REQ Request Target 50 I/O Input/Output Target ______________________________________
A typical single-channel SCSI cable connector of the prior art is comprised of a flat ribbon cable of fifty (50) wires in width. Every other wire of the fifty (50) wire flat ribbon cable is connected to ground to eliminate cross-talk between the adjoining wires that carry the data and control signals. The flat ribbon cable is typically enclosed in a low-cost insulator, such as polyvinyl chloride (PVC). The fifty (50) wire flat ribbon cable is terminated on each of its ends in a two (2) row by twenty-five (25) pin cable connector. The pins or elements of the SCSI cable connectors are spaced apart from each other in a given row on one hundred thousandths (0.100) inch spacings. Each of the pins of the two (2) row by twenty-five (25) pin SCSI cable connectors are preassigned one of the different single-ended SCSI bus signals as defined in the table above.
The SCSI bus can communicate on one end with one, or more than one, host computer or controller and on the other end can communicate with up to eight (8) peripheral devices. If the host is internally SCSI compatible, the bus may be directly connected to the host. Otherwise, a host adapter or SCSI controller may be provided between the one or more host computers and the SCSI bus to communicate using the SCSI protocol on one end and to communicate with the one or more host computer bus on the other end. To adapt the peripherals to the SCSI bus, a SCSI bus controller is used. The controller communicates the SCSI protocol on one end and communicates with the peripherals on the other end.
The SCSI bus controller typically includes an interconnection board having a plurality of connectors, a two (2) by twenty-five (25) SCSI cable connector being required for each of the peripherals to be controlled by the bus controller. In many large computer systems, thirty-two (32) SCSI buses are employed to connect one hundred twenty-eight (128) SCSI controlled peripherals such as disk drives to the host computer. Since each drive requires a corresponding dedicated connector on the bus controller interconnection board, (thus 128 connectors) it is desirable to provide a means and method for reducing the number of SCSI connectors required to support an array of SCSI peripherals, thereby greatly reducing the amount of board space ("real estate") required to support such a large number of peripheral devices.